In human race and primates, uric acid which is an organic acid is a final metabolite in purine metabolism in cells, and is excreted mainly from the kidney. In species other than the human race and the primates, it is metabolized to allantoin by an action of uricase in liver, and is excreted from the kidney. Therefore, for the other mammals, it seems that effects of dynamic abnormality of uric acid which is an intermediate product in the kidney on living body are small. Losing the action of uricase in the evolution process seems to be a cause of the fact that the human race has suffered from gout due to hyperuricemia since ancient times.
In humans, when is caused the decrease of uric acid excretion in the kidney causes hyperuricemia, the gout develops at high percentage, which becomes a risk factor for cardiovascular diseases and hypertension. On the other hand, it has been known that the increase of uric acid excretion in the kidney causes renal hypouricemia. Although abnormality of uric acid kinetics is not obvious in these diseases, it has been supposed that urate transporters in the kidney are deeply involved.
The uric acid kinetics in the kidney has been studied by experimental systems using a removed organ perfusion method and an isolated cell membrane vesicle system. In humans, it has been demonstrated that uric acid freely passes through renal glomerulus and thereafter mechanisms for reabsorption and secretion exist in proximal convoluted tubule. However, by the conventional technique, it has been difficult that urate transport system via cell membrane is analyzed in detail, and it has been desired that the transporter per se is isolated and analyzed.
It has been known that there is a remarkable difference among species in the urate transport in the kidney, and there exist the species where secretion is dominant such as swine and rabbit and the species where the reabsorption is dominant such as human, rat and dog. The swine of the species with secretion dominance excretes from 200 to 300% of uric acid per unit nephron, whereas a human of the species with uric acid reabsorption dominance excretes only about 10% of uric acid per unit nephron. Also, it has been known that responses to uricosuric accelerators and uricosuric inhibitors are different even among the species with reabsorption dominance. Accordingly, since the kinetics of uric acid and the responses to drugs in the kidney are different depending on the species, and uric acid is reciprocally transported, it has not been easy to isolate a molecular entity of the urate transporter though its existence has been assumed.
Among the urate transporters in the kidney, the transporters which reabsorb uric acid from renal tubular lumen have been studied for long time by the experimental system using the isolated cell membrane vesicle system. For the drugs currently used for the patients with hyperuricemia and gout, it is assumed that the transporter which reabsorbs uric acid in the kidney is inhibited. Also, it is forecasted that renal hypouricemia is caused due to gene aberration of this transporter.
Recently, it has been demonstrated that the transporters involved in the reabsorption of uric acid are exchange transporters of uric acid and various anions in several experiments. For pyrazinamide used as the first-line drug of antituberculous drugs at present, it has been shown that pyrazine carboxylate which is the metabolite of pyrazinamide is an exchange substrate of this exchange transporter and facilitates the reabsorption of uric acid. That is thought to be the cause of hyperuricemia frequently observed in the patients administered the antituberculous drug.
Accordingly, the transporter involved in the reabsorption of uric acid in the kidney is thought to play an important role for internal kinetics of uric acid. It has been anticipated that elucidation of its molecular entity leads to elucidate a mechanism of action of uricosuric accelerators and a cause of renal hypouricemia, and development of new gout curative medicines.
We have previously isolated and reported organic anion transporters, OAT1 (organic anion transporter) (Sekine, T. et al., J. Biol. Chem., 272:18526-18529, 1997), OAT2 (Sekine, T. et al., FEBS Letter, 429:179-182, 1998), OAT3 (Kusuhara, H. et al., J. Biol. Chem., 274:13675-13680, 1999), and OAT4 (Cha, S. H. et al., J. Biol. Chem., 275:4507-4512, 2000) which play central roles in medicament transport in the kidney, liver, brain, placenta and so on. These transporters belonging to OAT family are the transporters capable of transporting many organic anions with different chemical structures, and also perform the transport of various anionic medicaments.
It was not obvious whether the urate transporter belongs to the known transporter family, but since uric acid is a dibasic acid having both pyrimidine structure and imidazole structure and is one of the organic anions, the possibility that the urate transporter phylogenetically belonges to OAT family was anticipated. In OAT family, since OAT4 exists at the side of renal tubular lumen in the kidney and the existence of the transporter involved in the reabsorption of uric acid is also assumed at the side of renal tubular lumen, it has been also anticipated that the transporter is phylogenetically similar to OAT4.
From these facts, we have anticipated that the urate transporter in the kidney belongs to the organic ion transporter family.